1. Field of Invention
The invention relates generally to circuits for detecting ground overcurrent conditions in a three phase electric power circuit, and, more particularly, to a circuit for detecting a ground current of a three phase electric power circuit which exceeds a predetermined percentage of the three phase load current level of this circuit.
2. Prior Art
In known ground overcurrent protective relaying circuits for a three phase electric power system, such as that disclosed in my U.S. Pat. No. 4,027,203, issued May 31, 1977, the minimum ground overcurrent pickup is a fixed current value, which must be selected to be greater than the ground currrent which will flow in the electric power circuit during maximum load current conditions and maximum relative unbalance between the three phase currents. Consequently, high resistance ground faults below this fixed minimum ground pickup current level can remain undetected by these known ground overcurrent protection circuits. Studies have indicated that about one-third of the ground faults on three phase electric power distribution lines are not cleared by any protective device on the line.